docs: def upgrade_plan_list(self, subid, params=None):
          ''' /v1/server/upgrade_plan_list
          GET - account
          Retrieve a list of the VPSPLANIDs for which a virtual machine
          can be upgraded. An empty response array means that there are
          currently no upgrades available.
  
          Link: https://www.vultr.com/api/#server_upgrade_plan_list
          '''
          params = update_params(params, {'SUBID': subid})
          return self.request('/v1/server/upgrade_plan_list', params, 'GET')